The present invention generally relates to tunable laser diodes and more particularly to a distributed feedback laser diode having a diffraction grating as reflection means.
In relation to the super-large capacity optical fiber telecommunication system of the next generation, the coherent optical telecommunication system has been studied intensively. In the coherent optical telecommunication system, the information is modulated on a coherent optical beam by frequency shift keying modulation or phase shift keying modulation and is transmitted along an optical fiber network in the form of a modulated optical beam.
The light, being an electromagnetic wave, has frequency components similar to radio waves. As the frequency of light is in the order of 200 THz, which is much higher than the frequency of radio waves, an enormous amount of information can be transmitted when the light is used as the carrier of information transmission.
FIGS. 1A and 1B show an example of the conventional optical telecommunication process using the ordinary amplitude modulation of the optical beam. In such an amplitude modulation of the optical beam, a drive pulse having a magnitude exceeding the threshold level of the laser oscillation as shown in FIG. 1A is applied to the laser diode, and the laser diode produces an optical output as shown in FIG. 1B in response to the turning on and turning off of the laser diode. Alternatively, a separate modulator for selectively passing the optical beam produced by the laser diode may be used. In this case, the laser diode produces an optical beam with a constant optical output, and the modulator absorbs the light in accordance with the information signal to be transmitted. As a result, an optical output similar to that shown in FIG. 1B is obtained.
In the optical telecommunication system based on such an amplitude modulation of the optical beam, however, the wave nature of the light is not fully utilized. As the light has, as already described, frequency components similar to the radio waves, it is expected that a more efficient optical telecommunication system can be achieved when the wave nature of the optical beam is exploited, for example, in the form of frequency modulation.
Meanwhile, it is known that there are laser diodes which can change the oscillation frequency in response to the modulation of the drive current. In other words, the frequency modulation of the output optical beam is made principally possible by using the laser diodes. Thus, the laser diode is expected to provide a simple and effective means for constructing a coherent optical telecommunication system wherein the frequency shift keying (FSK) or phase shift keying (PSK) is employed.
In the coherent optical telecommunication system using a coherent optical beam produced by the laser diode, it is possible to use the so-called heterodyne or homodyne technique for reproducing the information from the transmitted optical beam. This heterodyne or homodyne technique is used commonly in conventional microwave or radio wave telecommunications, including the ordinary television or radio set. In the heterodyne detection of the information, a received signal is mixed with a local signal produced by a local oscillator and an intermediate frequency signal is extracted after suitable filtering as a beat caused by the interference of the received signal and the local signal. More specifically, when a signal having a frequency of f1+/-.DELTA.f1 is received and mixed with a local signal having a frequency f1 from the local oscillator, an intermediate frequency signal .DELTA.f1 is obtained as a result of interference between both signals. By using a number of central frequencies f1, f2, f3, f4, . . . in correspondence to a number of frequency channels, it is possible to send a large amount of information in accordance with the frequency division multiplex procedure. In the reception side, one can selectively reproduce the desired information signal from a selected channel by setting the frequency of the local oscillator suitably.
In such a coherent optical beam telecommunication system, the laser diode used in the transmission side is expected to change the oscillation frequency freely about a suitably chosen central frequency f1. In other words, a tunable laser diode has to be used for producing the frequency modulated optical beam. Thereby, it is desired that the optical output power not change with the change of the oscillation frequency. Further, it is desired that the optical beam produced by the laser diode have a sharply defined spectrum. When the spectrum of the produced optical beam is broad, a relatively wide frequency band is needed for each channel and associated therewith, a large frequency shift is required for the oscillation frequency of the laser diode. Further, associated with the broad spectrum of the output optical beam, the number of channels which can be secured at the time of a telecommunication made in the frequency division multiplex mode is decreased.
In laser diodes, the change of the oscillation frequency is caused in response to the change of the output optical power of the laser diode, as the change of the oscillation frequency is caused in response to the modulation of the refractive index of the resonator of the laser diode which in turn is caused in response to the modulation of the drive current. When the output power is changed in response to the frequency change, such a change of the output power or amplitude produces a side band of which the frequency changes in response to the modulation signal, and such a side band acts as a noise to the transmitted information.
Meanwhile the distributed feedback (DFB) type laser diode is known conventionally as a laser diode which is capable of changing the oscillation frequency relatively in a wide frequency range. FIGS. 2A and 2B show an example of the conventional DFB laser diode having a single electrode, wherein FIG. 2A shows the transversal cross-section and FIG. 2B shows a longitudinal cross-section.
Referring to the longitudinal cross-section of FIG. 2B, the laser diode has a buried structure including an n-type InP substrate 201, a diffraction grating 202 formed thereon, an n-type GaInAsP waveguide layer 203 provided on the grating 203, an intrinsic type active layer 204 of GaInAsP provided on the waveguide layer 203, and a clad layer 205 of p-type InP provided on the active layer 204. Further, an electrode 206 and an electrode 208 are provided respectively on a top surface of the clad layer 205 and on a bottom surface of the substrate 201. Thereby, there is formed a diode structure having the p-type region 205 and the n-type region 203 across the active layer 204. Further, the layers 203 and 205, having a lower refractive index, form together with the active layer 204 having a higher refractive index and sandwiched therebetween, an optical confinement structure for confining the optical beam in the active layer 204.
Referring now to the transversal cross-section of FIG. 2A, the active layer 204 and the waveguide layer 202 form together with a part of the substrate 201 a mesa structure, wherein a p-type buried layer 211 of InP supports both sides of the mesa structure laterally. In the p-type buried layer 211, there is provided another buried layer 212 of n-type InP such that a p-n junction 13 is formed along the boundary between the layer 211 and the layer 212.
At the p-n junction 13, and thus at both sides of the mesa structure, there is formed a depletion region which prohibits the invasion of carriers, and there occurs a concentration of carriers along a current path passing through the active layer 204. Thereby, an effective injection of the carriers in the active layer 204 is achieved. Further, the buried layer 211 acts also as the optical confinement layer for laterally confining the optical beam in the active layer 204. Thus, there occurs a laser oscillation in response to the injection of a current from the electrode 206 toward the electrode 208.
In such a conventional DFB laser diode, it is known that there occurs a change of the oscillation wavelength in response to the drive current injected via the electrode 206.
FIG. 2C shows a typical example of the frequency versus modulation characteristic of such a conventional DFB laser diode, wherein the efficiency of modulation defined as the frequency shift in response to the unit amplitude change of the amplitude modulated drive current is plotted against the frequency. As can be seen from FIG. 2C, the efficiency of modulation decreases significantly in the frequency range between about 10 MHz and 100 MHz. Such a change of the efficiency of modulation with the frequency causes a complicated problem in the frequency modulation of the optical beam produced by the laser diode.
The decrease of the efficiency of modulation in the intermediate frequency range as shown in FIG. 2C is believed to be caused as a result of cancellation of the thermally induced modulation effect which is predominant in the frequency range below about 100 MHz and the modulation effect caused by the interaction of the carriers and photons which is predominant in the frequency range above about 100 MHz. More specifically, the effect of the thermally induced modulation causes a red shift of the oscillation wavelength toward the longer wavelength side when there is an increase in the driving current, as such an increase of the driving current causes an increase in the refractive index. On the other hand, in the case of the modulation caused by the interaction of the carriers and the photons, the increase in the carriers in the active layer due to the increase in the driving current induces a decrease of the refractive index and there occurs a blue shift of the oscillation wavelength toward the shorter wavelength side.
On the other hand, there is another type of known DFB laser diode which can shift the oscillation wavelength widely and stably in response to the drive current and is still capable of providing an optical output with a sharply defined spectrum. In this type of the laser diode, the electrode at the top surface of the clad layer 205 is divided into a number of electrode segments (Y. Yoshikuni et al., Broad Wavelength Tuning Under Single-mode Oscillation with a Multi-electrode Distributed Feedback Laser, Electronics Letters, vol. 22, Oct. 23, 1986).
FIG. 3 shows a typical example of such a prior art multi-electrode DFB laser diode, wherein the laser diode has a structure similar to that of FIG. 1 except that the electrode 206 is divided into a plurality of electrode segments 206a and 206b. In operation, the ratio of a drive current Il which is injected via the electrode segment 206a with regard to a drive current I2 which is injected via the electrode segment 206b is changed. In response thereto, there is induced a change in the carrier concentration and associated therewith, a non-uniform distribution of the light intensity appears in the active layer 204 as illustrated in FIG. 3. Such a non-uniform distribution of the light intensity induces the localized change in the carrier density which in turn induces a corresponding change in the refractive index in the active layer 204 as well as in the waveguide layer 203. Such a change in the refractive index induces a change in the effective pitch of the diffraction grating, and in response thereto, there occurs a change in the oscillation frequency. It should be noted that the non-uniform distribution of the carrier density induces not only the change of the refractive index in the active layer but also a non-uniform distribution of the gain. Thereby, the condition of the laser oscillation with respect to the amplitude and the phase of the optical radiation may be changed in response to the non-uniform distribution of the carriers. Thus, such a non-uniform distribution of the refractive index and the gain combined together cause a change in the threshold carrier density which corresponds to the overall number of carriers in the active layer and as a result, there occurs the foregoing change in the oscillation wavelength and the optical output of the laser diode.
FIG. 4 shows an example of the frequency shift of the oscillation wavelength in response to a parameter R defined as R=I1/(I1+I2) wherein I1 stands for the current injected via the electrode segment 206a and I2 stands for the current injected via the electrode segment 206b. As can be seen in FIG. 4, the oscillation wavelength is decreased, as compared to the case of R=0.5, when the current I1 is decreased (R=0.2), while it is increased when the current I1 is increased (R=0.7). Further, the laser diode of FIG. 3 has a preferable feature such that the produced optical beam has a sharply defined spectrum with a width of several tens of MHz and the laser diode can provide an optical power of several milliwatts.
However, the conventional DFB laser diode shown in FIG. 3 has a serious problem in that the optical output power changes in response to the oscillation wavelength when the parameter R is changed by simply changing the current I1 and I2 independently. In order to obtain the frequency shift of the laser oscillation while maintaining a constant output power, it is necessary to change the current I1 and I2 simultaneously. It should be noted that the control of the current I1 and the current I2 is a complicated process as such a change induces both a frequency shift and a change of the optical output power and there is needed a complex control system for achieving the desired frequency modulation without causing a modulation of the amplitude.
Further, the conventional DFB laser diode has a problem in that it can provide only a limited frequency range in which the efficiency of modulation remains substantially constant. For example, the DFB laser diode of FIG. 3 can provide a frequency range of only about 1 GHz in which the modulation characteristic of the laser diode is substantially flat, while a frequency range of about 10 GHz or more is needed in the actual optical telecommunication system.